Stroke is one of the leading causes of morbidity and mortality in developed countries. A stroke occurs when blood vessels in the brain are compromised. It has been defined as a “sudden neurological shortage presumed to be of vascular origin” (translated from Bousser MG: Classification et orientation générales du diagnostic. In “Accidents vasculaires cérébraux”: Bogousslavsky J, Bousser M G, Mas J L, 1993, 95-103, Doin éditeurs.)
There are two main types of stroke, namely hemorrhagic stoke and ischemic stroke. Hemorrhagic stroke is relatively infrequent, and occurs when a blood vessel ruptures, flooding a portion of the brain with blood. On the other hand, ischemic (or nonhemorrhagic) stroke is much more common and occurs when a blood vessel is blocked (e.g., due to a clot), causing cerebral nervous tissue to be deprived of oxygen, ultimately leading to necrosis if left untreated.
Ischemic stroke is typically treated by administration of a thrombolytic, namely, a drug that aims to dissolve the clot that caused obstruction of the vessel in question. This technique restores blood flow to hypoperfused areas, potentially salvaging those portions of the affected cerebral tissue that have not yet been irreversibly damaged because they have been receiving blood flow from collateral arteries anastomosing with branches of the occluded vascular tree. These portions are known as the “ischemic penumbra”, while portions of the cerebral tissue that have been irreversibly damaged due to oxygen deprivation are known as the “core ischemic zone”. Further information can be found on the Internet by visiting the Internet Stroke Center at Washington University in St. Louis (http://www.strokecenter.org/education/ais_pathogenesis/22_ischemic_penumbra.htm).
The success of thrombolysis (i.e., treatment with a thrombolytic) therefore relies on rapid administration of the drug to a patient having undergone an ischemic stroke. With cerebral tissue being depleted at a rate of several (approximately 4) million neurons per minute, clinicians find themselves operating within a narrow time window (typically considered to be only a few hours) from occurrence of a stroke if there is to be any hope of success. If a thrombolytic is administered to a patient within this time window, then it may be possible to salvage the ischemic penumbra. However, reperfusion of severely hypoperfused areas can result in hemorrhage with its associated complications. If it turns out that the patient had insignificant salvageable cerebral tissue to begin with (i.e., with dim prospects for a positive outcome from the outset), then administering the thrombolytic will unnecessarily raise the risk of harming the patient.
For these reasons, the decision to administer a thrombolytic is not automatic. Rather, it is made as a function of whether the ischemic penumbra is large enough so as to justify pharmacological treatment and its attendant risk of hemorrhage. Accurate and rapid assessment of this risk/benefit scenario, among other factors, is therefore crucial in the early minutes of treating a patient with stroke symptoms.
As part of the decision making process, clinicians must typically assess the size of the ischemic penumbra and core ischemic zone by viewing and considering numerous images obtained from radiological instruments such as magnetic resonance imaging (MRI) machines. Due to the disparity in image type and presentation, it is only the most highly experienced clinicians who are able to extract useful diagnostic information from these images towards the decision to administer a thrombolytic. However, the number of clinicians having the requisite level of experience is expected to become inadequate to meet the needs of a growing and aging population that naturally brings about an increase in ischemic stroke cases.
As a result, improved image display and processing tools are needed to facilitate the diagnostic process, allowing a broader range of clinicians to assess the costs and benefits of administering a thrombolytic to a patient affected by an ischemic stroke.